1. Field of the Invention
The present invention relates to a lead frame and its method of manufacture, and is particularly concerned with a lead frame used for manufacturing a semiconductor package with single-sided encapsulation, such as an SON (Small Outline Non-Leaded) or a QFN (Quad Flat Non-Leaded).
2. Description of the Related Art
There is increasing use of chip-size packages for semiconductor devices mounted on a printed circuit board or the like due to market demand along with downsizing and miniaturization of electronic devices. In particular, a semiconductor package with single-sided encapsulation such as an SON package or a QFN package is commercially produced for semiconductor device that uses a lead frame for its manufacture. The semiconductor package with single-sided encapsulation uses resin that encapsulates or encloses a semiconductor chip or die placed on an upper surface of the lead frame, while exposing a portion of the lead frame along a lower surface of the lead frame, i.e., a back surface of the package.
FIG. 8 is an example of such semiconductor device 9 with single-sided encapsulation, which is manufactured by using a lead frame 5. The lead frame 5 is formed to have its pattern by etching or stamping (pressing) a base material 1 of a metal sheet. The patterned base material 1 is then plated to have a three-layered plating film consisting of an Ni layer 2, a Pd layer 3 and an Au layer 4 over the entire surface of the base material 1, i.e., a front surface 1a, a back surface 1b, and a lateral surface 1c. 
Semiconductor chips 6 are then placed on respective die pad sections 5a of the lead frame 5. Each semiconductor chip 6 is connected to the lead sections 5b of the lead frame 5 by bonding wires 7. An upper surface (front surface 1a) and a lateral surface 1c of the lead frame 5 are covered with resin 8 to encapsulate the semiconductor chips 6 mounted on the front surface 1a. This encapsulated unit is then cut apart or singulated to produce multiple individual semiconductor devices 9.
The semiconductor device 9 with single-sided encapsulation has a structural feature whereby lower surfaces (back surface 1b) of the lead sections 5b are exposed from the resin 8 so that they come into contact with an external board such as a printed circuit board (not shown). This feature however suffers from insufficient adhesion between the lead sections 5b and the resin 8, which leads to a problem in that some lead sections 5b become detached from the resin 8 and falloff during the cutting step described above.
To solve this problem, Japanese Patent Application Laid-Open No. 2006-93559 proposes a method of using two kinds of Ni-plating solutions to form two kinds of Ni layers, each having different compositions on the upper and lower surfaces of the lead frame. By forming these two kinds of Ni layers, a three-layered plating film having a rough surface and consisting of an Ni layer, a Pd layer and an Au layer is formed on the upper surface of the lead frame. On the other hand, another three-layered plating film having a smooth surface and consisting of an Ni layer, a Pd layer and an Au layer is formed on the lower surface of the lead frame. This configuration improves adhesion between the resin and the lead frame.
To evaluate the adhesion between the resin and the lead frame having the three-layered plating film with a rough surface, the adhesive strength was measured in the following manner. First, a three-layered plating film having the rough surface described above was formed on a metallic base material. On this three-layered plating film, four resin moldings each 2 mm in diameter were formed under a mold-filling pressure of 100 kg/cm2 and the molding conditions of 175° C.×90 seconds. These four resin moldings were then hardened for eight hours at approximately 175° C. in an oven. The four resin samples evaluated were thus formed. Each resin sample was then pushed sideways by applying an increasing load. The load value applied at the instant that the resin sample became detached was measured. Each load value thus obtained was divided by the area of adhesion of the corresponding resin sample to determine the load value per unit area. The average of these four converted values represents the adhesive strength between the resin and the lead frame.
This evaluation revealed that the adhesive strength between the resin and the metallic base material having the three-layered plating film with the rough surface was 19.9 MPa. For comparison, another metallic base material was prepared on which a three-layered plating film having a smooth surface and consisting of an Ni layer, a Pd layer and an Au layer was formed. The adhesive strength of this conventional three-layered plating film was evaluated in a similar manner, and was found to be 9.5 MPa. These results show that the three-layered plating film having a rough surface improves adhesion and has greater adhesion than the conventional three-layered plating film.
Japanese Patent Application Laid-Open No. 2006-310397 shows a technique for roughening the surface of a base material for the lead frame of a copper system. Specifically, this technique uses a micro-etching solution to slightly dissolve the metallic surface of the base material. In this manner, minute concavities and convexities are formed on the metallic surface of the base material.
On this roughened base material, a conventional three-layered plating film consisting of an Ni layer, a Pd layer and an Au layer was formed. Thereafter, the adhesive strength was evaluated in a manner similar to that described above. The adhesive strength of this roughened base material with the conventional three-layered plating film was 11.8 MPa. It was thus confirmed that forming the three-layered plating film on the roughened base material also improves the adhesion and has greater adhesion than the conventional three-layered plating film formed on the conventional metallic base material, although its adhesive strength was slightly inferior to that achieved by the technique shown in Japanese Patent Application Laid-Open No. 2006-93559 mentioned above.
The method of forming the Ni layers disclosed in Japanese Patent Application Laid-Open No. 2006-93559 may however cause a problem of warping due to the stress difference between the Ni layers, which are sequentially formed on each of the front and back surfaces of the lead frame using two kinds of plating solutions. Moreover, use of the two kinds of plating solution needs longer plating apparatus and more plating steps. Accordingly, the plating process requires complicated management and more processing time, which leads to low productivity. The micro-etching process shown in Japanese Patent Application Laid-Open No. 2006-310397 also has a problem that the etching process requires additional etching apparatus and more processing time, which also leads to low productivity.